leveling plywood floor before hardibacker is installed + confirm

bobafettucciniNovember 13, 2010

Hi, thanks again to Mongo for great answer to my previous questions!

I have a couple of questions about the floor. The area is 5x8, and I pulled up the old tile which was mortered to a very thin layer of plywood that had been stapled to the subfloor of plywood (looks like chipboard).

I plan to install 1/4 hardibacker to the subfloor before tiling with porcelain 12x12 mixed with 6x6 in a pattern.

The problem is that the subfloor dips about 1/4 inch along the egdes of two opposing walls. Also, in the middle of the area where the vanity cabinet will sit, there is a seam in the plywood that is uneven. The builder had left this portion untiled since it couldn't be seen.

I need advice on how best to flatten out this floor. I understand that is doesn't really have to be "level", just uniformly flat so the tile won't get stressed and crack.(By the way the old tile was 4x4 and had been there 11 years with no cracks).

1. If I leave the under-cabinet area untiled, do I need to fix the floor dip on the other wall?

2. Assuming the dip needs to be fixed, can i glue shims to the sub floor?

3. Can I use self leveling compound? The instructions for LevelQuick say it can used only for exterior grade plywood, but not particle board. What about my "chip-board" plywood? Can it be primed first, then use SLC? Would the SLC run under the wall? Any other issues with using this stuff?

4. Could I use a small amount of the thinset that will be used between the plywood + hardibacker instead of SLC, and use it to level out the subfloor dips, then let it dry/cure, then thinset and install the hardi later?

5. From reading other posts I want to confirm that UNmodified (dry-set)thinset should be used between the plywood + hardibacker. (Both Mongo + Bill recommend, right?)

6. Once hardibacker is in place, do I use modified or unmodified thinset for installing the porcelain tile?

7. On the shower walls, I will also use hardi, and same tile. However, I want to put in a 4 inch stripe of glass tile running through the field. What do you recommend for the mortar/thinset for this tile? Home depot has something labeled for glass I assume would be ok.


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I don't have a good enough grasp on 1) and 2) to comment. Photos would help. Regardless, the idea is as you wrote, get the floor FLAT. Level is nice, but not required. Flat is important, even more so when you use larger tiles.

3) Yes, SLC can be fine. But before using, I'd take out the tile (done) and the "very thin layer of plywood" under the tile (if not done). IF the subfloor underneath is fine, then add 3/8" or 1/2" plywood over that, screwed. That should give you a nice 1-1/8" minimum of structure under the tile. Then you can SLC over that if appropriate. It might not be appropriate if, for example, the doorway into the bath is the low point in the room. You might not want a half-inch lip of SLC at the threshold.

SLC will run wherever you let it. Caulk, tape, or somehow seal or dam up wherever you don't want it to go. Think of it as water. If there's a crevice or passageway available, it'll flow there.

Best to have two people, one mixing and the other placing. You want to get it all down NOW and help it where it needs to be helped, then let it self-level.

4) Yes. Just remember that thinset is THINset, not THICKset. If you need a thick build, then use a medium bed mortar, or just mix your own, portland cement an sand.

5) You can use whatever you want, unmodified is used because it's simply less expansive. You don't need a strong bond to hold the cement board down. That's what the screws do. The thinset is simply a gap-filler, it prevents vertical movement between the cement board and the plywood. So unmodified or modified will work.

6) Modified.

7) I use white modified thinset. White is important if the glass is translucent. Gray will muddle the color of the glass. Watch ridge marks too. Uncompressed ridges/valleys can telegraph through some glass tiles.

    Bookmark   November 14, 2010 at 1:16AM
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Thanks Mongo again for the quick reply.

If I use thinset as my leveler, then let it dry, then thinset and screw down the hardi, will i have a problem getting the screws thru the already set layer of thinset?


    Bookmark   November 14, 2010 at 11:19AM
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Depends on how thick the thinset is. If thick, drill pilot holes with a proper $3 bit. That'll help wth two things; the screws getting through the material, and it'll minimize the chance of the screws breaking up and fracturing the feathered layer of thinset as they try to displace their way through it.

A consideration when sandwiching a feathered or leveling layer of thinset or SLC between two materials (subfloor and cement board for example) is that the screws can fracture and loosen the feathered layer of thinset/SLC.

Regardless, when they then walk on the floor, movement has caused some of them to get a crunchy granola sound. Now I've never had that happen, but I've heard complaints here on the forums from people that it has happened to. It's normally from people doing something like SLC to level, then putting another material (cement board for example) over the cured SLC and screwing through that sandwich. I never really understood that method of work in the first place, as you can tile on the SLC.

In your case though, where you're doing a feathered area of thinset, then more wet thinset when you set the cement board, I don't think that'd be a problem as the wet thinset will bond and consolodate the broken up cured thinset. If it does indeed break up.

Really, it's one of those "maybe" considerations, but I though I'd throw it out there as a caution. And obviously use longer screws if your leveling layer of thinset is thick.

    Bookmark   November 14, 2010 at 12:29PM
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OK, sounds like the easiest solution.
Thanks again for the good advice!

    Bookmark   November 14, 2010 at 12:33PM
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Hi I have noticed a number of different combinations before setting tiles.
I have tongue and groove floor boards.
1.) I am planning to use a levelling 3/8 ply over the tongue and groove screwing it to the floor boards but not the joists.
2.) Applying hardiebacker boards with thin set and screwing using hardiebacker screws flush.
3.) Fibre taping and mudding staggered joints.
4.) Laying electric underfloor heating (continuity test wiring first) securing and then thin SLC over the top (silicone edges to stop it running away)
5.) tile onto of set SLC using decent poly modified flexible thin set

My questions are:
Anyone have any issues I may have missed with method above.
Should I lay a layer of thinnest between tongue and groove floor boards and levelling ply.
I am planning to prime plywood surfaces and edges and tongue and groove boards.
I would really appreciate comments

    Bookmark   December 14, 2014 at 3:30PM
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You'll get more attention if you post a new thread on this. Many people don't open really old threads.

    Bookmark   December 15, 2014 at 2:31PM
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